Digital equipment is highly susceptible to noise in the power source. As a consequence, filters are normally interposed in the power line, and it is of concern to the filter manufacturers or equipment manufacturers that the effectiveness of their filters be ascertained. The device of the present invention serves to inject a transient or noise into such a circuit to determine the efficacy of the filter.
It is desirable that testing procedures achieve as nearly as possible the practical conditions arising in the field use of digital equipment. To this end, the output impedance of the generator should be close to what the power line impedance would be. While power line impedances are not particularly consistent as between various areas, their impedance is generally low.
A transient generator should generate a transient having a broad continuous band of frequencies running from 100 Hz upwardly of 100 MHz to be certain that the filtering system accommodates any frequency within that spectrum.
As far as is known, pulse generators have invariably been transformer-coupled to the equipment undergoing tests. A capacitor is charged by a large impedance power supply. An SCR is connected across the capacitor to effect its discharge, and the primary of a step-up coupling transformer is connected to receive the pulse of the discharging capacitor, the secondary of which is connected either in series or parallel to the equipment undergoing tests and amplifies the voltage to a maximum output of, for instance, 600 v.
The transformer coupling has two undesirable effects. First, the pulse rise time is relatively slow which results in a diminished spectrum of noise at the high frequency end, as may be predicted by Fourier analysis. Second, the output impedance of the transient rises sharply with the frequency. For example, a characteristic transformer-coupled pulse generator which shows a 4 ohm output impedance with respect to the 100 Hz component of the transient will show about 500 ohms with respect to the 25 MHz component. Thus simulation of power line conditions at the high frequency end of the transient is defeated.
Transient generators as hitherto known have been generally of the relaxation oscillator type wherein circuit elements in the oscillator are adjusted to obtain the desired periodicity of the transient pulses or the desired phase angle thereof. This requires the use of external instrumentation such as oscilloscopes and frequency counters along with the transient generator in order to establish the frequency or the phase angle.